1. Field of the Invention
This invention relates generally to optical communications systems, and more particularly to signal regeneration in optical repeater systems.
2. Description of the Related Art
In contemporary communication systems operating at ever-increasing transmission rates, return-to-zero (RZ) signaling has become a popular method for data exchange. In RZ signaling, a strong component of the clock frequency exists in the data spectrum, providing a reference on which a phaselocked loop (PLL)-based, or filter-based, clock recovery unit (CRU) can latch in order to recover the clock signal from the received data stream. Such PLL-based CRU systems are well studied and characterized in the technical literature.
At relatively low data transfer rates, a fundamental oscillator is commonly deployed in the PLL, and the recovered clock is obtained directly. At higher frequencies, fundamental high-Q oscillators are difficult to build, and are therefore costly, due to the reduction in resonator size. While a frequency doubler can be used, a more compact and relatively low power approach is to employ a harmonic mixer as the phase detector for the phaselocked loop.
Transmission of data over long fiber links results in timing jitter, which is a major signal degradation problem. The timing jitter refers to a random variation of the pulses"" arrival time at the receiver, and it can be on the order of a pulse width. Often there is also a slow variation in the propagation time of the transmitted data caused by the change in the fiber refractive index due to temperature variation and mechanical disturbance. The jitter may cause slight changes in the repetition rate of the data at the receiver. In packet switching systems, the arrival time of the data packets is stochastic. Therefore, at a node or a receiver the timing of the data needs to be extracted and synchronized with a local clock that in turn drives the data processing devices, which perform other signal processing operations, such as optical demultiplexing or 3R (re-amplification, retiming, and reshaping) optical signal regeneration. This extraction of the data timing at the receiver is called clock recovery. As optical data transmission speeds approach 100 gigabits/second, a phase noise yielding less than 1 picosecond of clock timing jitter is required.
As described above, PLL circuits have been used in conventional clock recovery systems. When used in optical communications networks they convert received optical signals to electronic signals and then back to optical signals for transmission. Because high-Q oscillators are difficult to build and the operation speed and performance of the conventional electric PLL circuits are limited by the response of the phase comparators used, new techniques based on photonic solutions are required for high-speed optical communications networks.
An object of the present invention is to provide an all-optical timing extraction device using the non-linear characteristics of optical waveguides for counter-propagating pulses.
Another object of the present invention is to provide an all-optical timing extraction device that can resolve timing delays between pulses on the order of a few picoseconds.
Yet another object of the present invention is to provide an all-optical timing extraction device that is bit rate flexible.
In order to attain the objectives described above, according to an aspect of the present invention, there is provided an optical clock recovery device using non-linear optical waveguides whereby relative arrival timing differences between optical data and optical clock pulses counter-propagating in a non-linear waveguide are determined. The extracted timing information is used in conjunction with a balanced photodetector to generate electrical error signal. This error signal is used in a phase-locked loop configuration to perform optical clock recovery.
This invention is based on the non-linear optical properties of semiconductor waveguides. Any non-linear semiconductor waveguide can be used. Herein two different types of non-linear semiconductor waveguides devices, one reverse biased, to increase the optical absorption coefficient, and one forward biased, to increase the optical gain coefficient, are used as exemplars. The first is an electro-absorption modulator (EAM) and the other is semiconductor optical amplifier (SOA). These devices are usually fabricated from InGaAsP-based material and can operate in a wide range of wavelength including the 1310 or 1550 nm telecommunication wavelength window. The EAM is usually reverse biased with a fixed dc voltage to increase the optical absorption coefficient inside its wave-guide. While a forward dc current is used to bias the SOA and increase the optical gain coefficient inside its wave-guide.
These waveguides have nonlinear transmission characteristics that can be controlled optically. When an intense optical control pulse propagates through the EAM, electron-hole pairs are generated from inter-band absorption. The electron-hole pair drifts apart from each other in responds to the internal dc electric field. As a result, an opposite space charge is established which reduces the internal dc electric field which in turns decreases the absorption in the waveguide. Therefore, the optical control pulse saturates the EAM absorption resulting in an increase In its transmission. Correspondingly, when an intense optical control pulse propagates through the SOA, stimulated emission diminishes the electron population and thus reduces the optical gain inside the waveguide. Therefore, the optical control pulse saturates the SOA gain resulting in a decrease in its transmission.
When two counter-propagating pulses with sufficient peak powers are injected into a nonlinear waveguide, they will both contribute to its transmission saturation. The amount of saturation will depend on the timing difference (delay) between the arrivals of the two pulses. The leading pulse, acting like the control pulse, saturates the loss and increases the transmission in the case of an EAM, or saturates the gain and decreases the transmission in the case of an SOA, seen by the lagging pulse. Therefore, the transmission of the lagging pulse is usually higher in the case of an EAM, or lower in the case of an SOA, than that of the leading pulse.
Given that the nonlinear device has two possible outputs, one for each counter-propagating pulse steam, a delay-dependent peak power transmission for each output is measured for the non-linear semiconductor wave-guide device. This phenomenon is well documented by E. S. Awad, C. J. K. Richardson, P. Cho, N. Moulton, J. Goldhar in xe2x80x9cOptical Clock Recovery Using SOA For Relative Timing Extraction Between Counter-Propagating Short Picosecond Pulses,xe2x80x9d IEEE Photon. Tech. Lett., vol. 14, no. 3, March 2002 and in xe2x80x9cBi-Directional Coupling In Nonlinear Waveguides ForAbsolute Timing Determination,xe2x80x9d presented at CLEO 2001 Technical Digest, Baltimore, Maryland, poster session CThL51, which are both hereby incorporated by reference in their entirety.
Based on this principle, two optical beams with the same or different wavelengths or polarizations are launched simultaneously into a, typically fiber-pigtailed, nonlinear semiconductor waveguide device in a counter-propagating configuration. The first beam, here the data, is a pulse train encoded with RZ binary digital information, typically using amplitude shift keying. The second beam, here the clock, is a stable pulse train produced by an optical short pulse source (OPS) with a repetition frequency close to that of the data and a very low timing jitter. The amount of transmission change, however, depends on the relative delay between the data and clock signals inside the device. Therefore, the transmitted power of the data and clock signal depends critically on the relative arrival timing of the two pulses inside the waveguide.
The transmitted clock and data optical signals are then split off and detected by a balanced photo-detector to measure the difference between the average optical powers of the non-linear semiconductor wave-guide device data and clock outputs, and produce an electrical error signal at the detector""s output. The balanced photodetector is slow enough to average the power over several pulses to detect an average clockxe2x80x94this is in effect a low-pass filter. The polarity of the generated error signal indicates which pulse stream advances the other, while the amplitude of the error signal indicates the amount of the delay between the two pulse streams. The error signal provides complete information about the timing error between the data and clock pulses. This timing error is then compensated for by proper tuning of the frequency and phase of the OPS.
This error signal may be provided directly to the OPS but in more typically configurations, this error signal is fed back through an electronic amplifier to the voltage-controlled oscillator (VCO) input to adjust its repetition rate. The VCO in turn drives the OPS to adjust its repetition frequency. The repetition rate and phase of the OPS stabilize only when the clock and the data are synchronized.